CuVoodoo #009 – taming glouttonous lions

building an electronic cat repeller to guard the fridge:
My sneaky cat can open all doors, even the fridge door. No meat is safe any more.
To prevent that I built a cat repeller using an Atmel ATmega328P Arduino Nano 3.0 board.
An HC-SR501 PIR motion detector checks if the cat is nearby. Then an E18-D80NK range detector verifies if the cat is in front of the fridge.
If this is the case a ~24 kHz PWM signal will generate an ultrasound using a piezoelectric diaphragm, encouraging the cat to leave the premisses.
If it still opens the fridge, a human audible alarm will sound until it is closed.